Intravenous administration of medicinal liquids has traditionally been accomplished using a drip-regulated gravity flow system. A more precisely regulated flow of the infused liquid generally can be achieved using a pump to advance the liquid at a predefined rate. For this reason, a peristaltic pump is often used to administer drugs intravenously through a continuous line that extends directly from a bag or bottle reservoir to the patient. The IV line of flexible tubing is threaded into a channel formed within the peristaltic pump body, and this section of the flexible tubing is compressed to displace fluid from the pump.
In one type of peristaltic pump, rollers are mounted on each end of a rotating arm. These rollers advance along the longitudinal axis of the flexible tubing as the arm rotates, thereby compressing the tubing and displacing a bolus of fluid from the pump ahead of each advancing roller.
Another type of positive displacement peristaltic pump includes a plurality of fingerlike plungers that are sequentially actuated to compress a section of flexible tubing, thus defining an advancing point of compression that forces fluid through the tubing and out of the pump. The plungers are typically actuated by a plurality of cams disposed along a motor driven shaft. An example of such a pump is described in U.S. Pat. No. 4,479,797.
The flow rate of a fluid being administered with a peristaltic pump is normally controlled by varying the speed of the rotating arm or the rotational speed of the cams that transfer the force to displace fluid in the flexible tubing. Changes in the volume of the flexible tubing that extends through the pump can significantly affect the rate that fluid is dispensed by the pump. The disposable tube sets employed to intravenously administer drugs typically comprise polyvinyl chloride (PVC). Continued flexure of PVC tubing eventually degrades its ability to elastically recover from the compressed state to its full uncompressed state as the compression force is removed. Even during its initial use in the pump, PVC tubing returns to its uncompressed shape relatively slowly. Thus, a change in the tube's elasticity over time can vary the volume of liquid advanced by a peristaltic pump during each pumping cycle or stroke. This variation in tubing elasticity or stiffness appears as an error in the rate at which fluid is delivered by the pump.
Tube shapers have been used in peristaltic pumps to force the flexible tubing to open up to its uncompressed shape and to compensate for the inadequate elasticity of the low cost PVC tubing sets usually used in peristaltic pumps. The tube shaper laterally squeezes the compressed tubing as the pumping compression force is withdrawn or reduced, to force the tubing to resume its round, uncompressed configuration. By restoring the tubing to its fully uncompressed state after each periodic pumping compression, the volume of the internal passage filling with fluid remains generally constant, even after extended use of the tubing in the pump. As an alternative to use of a tube reshaper, silicon tubing, which has much better elastic properties than PVC tubing, can be used within a peristaltic pump, but this approach adds substantially to the cost of the disposable tubing set.
In a copending, commonly assigned U.S. patent application entitled, "Volumetric Pump With Spring-Biased Cracking Valves," Ser. No. 07/742,623, filed 8 Aug. 1991, a novel type of positive displacement volumetric pump is disclosed that provides a much more accurate and constant rate of flow than can be achieved using a conventional peristaltic pump. The volumetric pump disclosed in this patent application displaces fluid from within a section of flexible tubing disposed between spring-biased inlet and outlet valves. The inlet valve is forced open by fluid at a predefined "cracking pressure," as the section of tubing is initially compressed, thereby permitting excess fluid within the compressed section to backflow toward a source container. The inlet valve then closes fully, and the outlet valve "cracks open" in response to the cracking pressure, enabling a predefined volume of fluid at the predefined cracking pressure, to flow from the pump to the patient.
Clearly, variations in tubing stiffness or elasticity could affect the cracking pressure and thus the accuracy of the rate at which this volumetric pump delivers fluid. Therefore, the effect of variations in the elasticity of the flexible tubing used in this novel volumetric pump is somewhat analogous to the effect of changes in the elasticity of such tubing when used in a conventional peristaltic pump.
Accordingly, to maintain an accurate rate of fluid flow from the novel volumetric pump briefly described above, it is an object of the present invention to compensate for variations or changes in the stiffness or elasticity of flexible tubing from which fluid is displaced when the tubing is compressed. It is a further object to provide a pump valve for use in the volumetric pump that is biased closed by a spring, but opens when fluid pressure in the tube reaches a predefined level, substantially independent of variations in tube stiffness or elasticity. Yet a further object is to vary the force acting to bias the pump valve closed to compensate for variations in the stiffness or elasticity of the flexible tube, so that a substantially constant pressure is required to force the valve open. Yet a further object is to provide a pump valve having a surface that applies a compression force to the tubing to control fluid flow therethrough, a fluid pressure within the tube acting on this surface to provide a force in opposition to the compression force, to force fluid through the tubing at a predefined pressure. These and other objects and advantages of the present invention will be apparent from the attached drawings and the Description of the Preferred Embodiment that follows.